1. Field of the Disclosure
The present disclosure relates to a liquid crystal display (LCD) device, and more particularly, to a uniform standing helix (USH) mode liquid crystal display device including at least one positive C plate.
2. Discussion of the Related Art
As information technology progresses, flat panel display (FPD) devices having light weight, thin profile and low power consumption have been developed. Specifically, a cathode ray tube (CRT) device has been replaced by the FPD devices such as a liquid crystal display (LCD) device.
The LCD devices use the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. Due to the optical anisotropy of the liquid crystal molecules, refraction of light incident onto the liquid crystal molecules depends upon the alignment direction of the liquid crystal molecules. The liquid crystal molecules have long thin shapes that can be aligned along specific directions. The alignment direction of the liquid crystal molecules can be controlled by applying an electric field. Accordingly, the alignment of the liquid crystal molecules changes in accordance with the direction of the applied electric field and the light is refracted along the alignment direction of the liquid crystal molecules due to the optical anisotropy, thereby images displayed.
Among various types LCD devices, an active matrix type liquid crystal display (AM-LCD) device in which a thin film transistor (TFT) and a pixel electrode are disposed in a matrix has been the subject of recent research due to its high resolution and superior quality for displaying moving images. The LCD device includes a color filter substrate having a common electrode, an array substrate having a pixel electrode, and a liquid crystal layer interposed between the color filter substrate and the array substrate. In the LCD device, the liquid crystal layer is driven by a vertical electric field between the pixel electrode and the common electrode. Although the LCD device provides a superior transmittance and a high aperture ratio, the LCD device has a narrow viewing angle because it is driven by the vertical electric field. Accordingly, various other types of LCD devices having wide viewing angles, such as in-plane switching (IPS) mode LCD device, have been developed.
FIG. 1 is a cross-sectional view showing an in-plane switching mode liquid crystal display device according to the related art. In FIG. 1, an in-plane switching (IPS) mode liquid crystal display (LCD) device includes first and second substrates 1 and 2 facing and spaced apart from each other and a liquid crystal layer 3 between the first and second substrates 1 and 2. The liquid crystal layer 3 includes liquid crystal molecules 5. A pixel electrode 7 and a common electrode 9 are formed on the first substrate 1. When a voltage is applied to the pixel electrode 7 and the common electrode 9, a horizontal electric field is generated between the pixel electrode 7 and the common electrode 9 to control the liquid crystal molecules 5.
In an OFF state, an electric field is not generated between the pixel electrode 7 and the common electrode 9, and liquid crystal molecules 5 keep an initial alignment state such that the IPS mode LCD device displays a black image. In an ON state, a horizontal electric field is generated between the pixel electrode 7 and the common electrode 9, and the liquid crystal molecules 5 are re-aligned along the horizontal electric field such that the IPS mode LCD device displays a white image.
Although the IPS mode LCD device has advantages in a viewing angle and a response speed, the IPS mode LCD device has disadvantages in a contrast ratio. The IPS mode LCD device has a low contrast ratio due to a light leakage in the OFF state. To improve the low contrast ratio of the IPS mode LCD device, a vertical alignment (VA) mode LCD device has been suggested.
FIG. 2 is a cross-sectional view showing a vertical alignment mode liquid crystal display device according to the related art. In FIG. 2, a vertical alignment (VA) mode liquid crystal display (LCD) device includes first and second substrates 11 and 12 facing and spaced apart from each other and a liquid crystal layer 13 between the first and second substrates 11 and 12. The liquid crystal layer 13 includes liquid crystal molecules 15. A pixel electrode 17 having a slit 18 is formed on the first substrate 11. In addition, at least one protrusion 20 is formed on the second substrate 12, and a common electrode 19 is formed on the at least one protrusion 20 and the second substrate 12. When a voltage is applied to the pixel electrode 17 and the common electrode 19, a vertical electric field is generated between the pixel electrode 17 and the common electrode 19 to control the liquid crystal molecules 15.
Although the VA mode LCD device has advantages in a contrast ratio, the VA mode LCD device has disadvantages in the viewing angle. Accordingly, an LCD device having advantages both in a contrast ratio and a viewing angle is desired.